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1.
In French Canadian slang, pretty much the equivalent of 'fuck' but with no sexual meaning - the word is only used as a vulgar interjection. Thanks to its ample use of the 'a' vowel, it is an extremely satisfying swear word that lets the energy flow through your throat.

The word comes from French 'tabernacle' pronounced with a French Canadian accent. It is unheard of in France.

Note: 'crisse', 'hestie' and 'câlisse' have very similar meanings and are equally vulgar. A locutor can stack them together as to create a rhetorical figure (see examples).
Tabarnac! - shit!

Hestie de câlisse de tabarnac! - fucking shit!

T'es un tabarnac de fif - ur fucking gay

Hestie de tabarnac de plotte - motherfucking cunt
by Mgr Turcotte February 24, 2006
 
2.
French slang (used in Canada only) In its literal sense, it means "tabernacle", but in this sense, it's used as a way of saying "Damn it"
Maman: "Jeanette, qu'est-ce que tu as acheté au mall?"
Jeanette: "J'ai acheté une robe pour 100$."
Maman: "Tabarnac! C'est trop cher!"
by Siena July 01, 2005